A Linear Quantum Coupler for Clean Bosonic Control

TL;DR

This paper introduces a novel linear quantum coupler that enables high-fidelity, selective bosonic control in superconducting circuits, reducing parasitic effects and decoherence for improved quantum operations.

Contribution

It proposes a new quantum mixer combining Kerr-free and balanced features with engineered selection rules, achieving near-linear idle behavior and clean parametric activation.

Findings

The mixer is essentially linear when idle.

It activates clean parametric processes at high drive strength.

The ideal Hamiltonian is analytically simple and robust against imperfections.

Abstract

Quantum computing with superconducting circuits relies on high-fidelity driven nonlinear processes. An ideal quantum nonlinearity would selectively activate desired coherent processes at high strength, without activating parasitic mixing products or introducing additional decoherence. The wide bandwidth of the Josephson nonlinearity makes this difficult, with undesired drive-induced transitions and decoherence limiting qubit readout, gates, couplers, and amplifiers. Significant strides have been recently made into building better `quantum mixers', with promise being shown by Kerr-free three-wave mixers that suppress driven frequency shifts, and balanced quantum mixers that explicitly forbid a significant fraction of parasitic processes. We propose a novel mixer that combines both these strengths, with engineered selection rules that make it essentially linear (not just Kerr-free) when idle, and activate clean parametric processes even when driven at high strength. Further, its ideal Hamiltonian is simple to analyze analytically, and we show that this ideal behavior is first-order insensitive to dominant experimental imperfections. We expect this mixer to allow significant advances in high-Q control, readout, and amplification.